5 minute read
Building a separate ADU in your backyard
Where you live decides this more than anything about your property. A detached unit is allowed by right in Alexandria, allowed in Arlington and Loudoun, and in Fairfax needs a special permit and a lot of at least two acres — which rules it out for most people there. It is also the most expensive route per square foot, because everything has to be built and every service run to it.
The rules vary more here than for any other type
Alexandria permits a detached accessory dwelling outright, and it may sit as close as one foot from your side or rear lot line. Loudoun permits one and allows up to 2,500 sq ft in its rural and transition districts. Arlington permits one but keeps it 25 feet back from the street and 8 feet from the house. Fairfax permits one only by special permit from the Board of Zoning Appeals, and only on a lot of at least two acres. Same project, four very different answers.
Fairfax § 4102.7.B(15)(a); Alexandria § 7-203(C); Loudoun § 4.02.01 · Read the sectionSetbacks usually decide where it goes, not the size limit
People start with square footage and discover the constraint is placement. A generous size allowance is no use if the only part of the lot it can legally sit on is the part with the mature oak, the septic field or the only access route. Work out where a building may go first, then how big it can be. That order saves a lot of redrawing.
Why it costs more
You are building everything: foundation, walls, roof, and a run of water, sewer and power across the yard to reach it. There is no existing structure to borrow from and no shared wall. Against that, you get a building shaped entirely around what you want rather than around what the house already is, and it does not disrupt the main house while it is being built.
Things that catch people out
Construction access, if the only way to the back is a narrow side passage. Mature trees, which several jurisdictions protect and which are expensive to work around. Where the sewer line actually runs, which is rarely where people assume. And whether the lot slopes enough to change the foundation, which can move the cost meaningfully before anything is above ground.
Questions people ask
Can I build a detached ADU in Fairfax County?
Only by special permit, and only on a lot of at least two acres. Fairfax requires an accessory living unit to sit wholly within the existing house unless the Board of Zoning Appeals grants a special permit, which it can do only on lots of two acres or more (§ 4102.7.B(15)(a)). For most Fairfax lots that means a detached unit is not currently available.
How close to the property line can it go?
It depends entirely on the jurisdiction. Alexandria allows one foot from side and rear lot lines, three feet where the facing wall has windows or doors. Arlington requires five feet at the sides and rear and 25 feet from the street. Loudoun allows five feet from side and rear lines where the encroaching part has no windows or doors.
Is a detached ADU more expensive than a conversion?
Usually, yes, per square foot. You are building a whole structure and running services to it rather than adapting something that already exists. The tradeoff is that the design is not constrained by an existing building, and the main house stays usable throughout.
Where could one actually go on your lot?
This is the question worth answering before anything else, and it is mostly about setbacks, services and access rather than square footage. Send us your address and we will tell you what your jurisdiction allows, roughly where on the lot a building could sit, and what it would cost to build there.
See what you can buildAnything here that quotes the law links to the section it came from, so you can read the original. Rules differ by jurisdiction — check what applies where you are. This is general information, not a determination about your property.